British defence experts expressed alarm yesterday after Donald Trump suggested he might undo a multi-billion dollar stealth fighter project that is to form a core part of the United Kingdom's defensive capabilities.

But defence experts told this newspaper there would be disastrous consequences for Britain if Mr Trump were to put the plan on the scrap heap at this late stage.

"If the US were to cancel the programme in its entirety tomorrow, the UK would truly be up a creek without a paddle," Francis Tusa, editor of Defence Analysis, said.

Financially, Britain would lose the £2billion it has contributed to the research and development of the supersonic jet, which has been designed to avoid detection on enemy radar.

And, Mr Tusa said, it would leave the UK without a plane to put on its new Queen Elizabeth-class aircraft carriers: "This is the only plane with vertical landing".

Liz Quintana, air power expert at the Royal United Services Institute agreed that if America decided to cancel the programme, it would leave Britain and other European allies who have placed orders "a bit stuck".

She said there was no alternative so-called fifth generation stealth fighter available anywhere else, or even on the drawing board.

She said: "The US Air Force would be as stuck as anyone else in terms of what it buys instead."

If America cut its order numbers, it would also drive up the overall cost of UK orders.

Britain has agreed to buy 138 of the fighters over the next two decades and to buy 24 by 2023.

Donald Trump is interviewed by Chris Wallace of Fox NewsCredit:
AP

Mr Tusa said scrapping the programme would also have far-reaching implications in foreign diplomacy.

Mr Trump's tweet came as Ash Carter, the defence secretary, travelled to Israel to attend a ceremony marking the delivery of the country's first F-35 fighters.

But both Ms Quintana and Mr Tusa agreed that, given the dramatic repercussions, they could not imagine the United States pulling out of the programme altogether.

Rather, Ms Quintana, interpreted Mr Trump's message as the "first shot" in a tough round of cost negotiations with Lockheed Martin, the Government’s largest defence contractor and developer of the programme. The British defence giant is the only “tier one” partner in the project, and produces about 15pc of each aircraft.

The F-35 project has been criticised for its cost and delay

The project - which is expected to cost $1.5 trillion dollars over its 50-year lifetime including in service support - has come under fire for being overly costly, slow and failing to meet initial expectations.

The price for the 2,450 aircraft the US military will fly has almost doubled from early expectations to $400billion. Initial contracts also expected there to be about 1,000 of the jets ready by 2016, but by summer the consortium behind the aircraft had delivered fewer than 200.

The F-35 has also come in for criticism that it is unable to “dogfight” as well as jets which are 30 years older than it, though the manufacturers say this is an unfair comparison as it was never designed for this.

Ms Quintana said: "Frankly everyone would benefit in the programme if we managed to bring the price down."

Mr Tusa pointed out that many countries involved in the programme, including the US, appeared to be slowly backing off their from original commitments of how many planes to buy.

Following Mr Trump's tweet, Lockheed Martin shares reportedly plunged by more than 4 per cent in early trade, shaving billions of dollars from the company's market value, while BAE’s stock fell almost 2.5 per cent.

Jeff Babione, Lockheed's F-35 chief, said the company understood concerns about affordability, but added: "It's an amazing programme. It's great value and I look forward to any questions that the president-elect may have."

Mr Trump's attack comes a week after he condemned as "out of control" the cost of a new Air Force One plane being built by Boeing.